Terrain Aided Navigation or TAN constitutes a particular means of navigation that can be applied to a wide variety of carrier vehicles, for example aircraft, submarines, autonomous missiles, etc.
There exist three main known means aimed at fulfilling the needs of carrier craft navigation. The first main known means comprises the inertial navigation techniques. The second main known means comprises the radio-navigation techniques. The third main known means comprises the navigation techniques using terrain correlation.
Inertial navigation consists in utilizing information supplied by inertial guidance systems. The operation of an inertial guidance system is based on Einstein-Galilée's principle of relativity, which postulates that it is possible, without the aid of signals external to a carrier craft, to measure, on the one hand, the speed of rotation of the carrier craft with respect to an inertial reference frame, for example defined by a geocentric reference associated with fixed stars and, on the other hand, the specific force applied to the carrier craft: typically its acceleration in the inertial reference frame, reduced by the acceleration due to gravity. A typical inertial navigation system, commonly denoted INS, is a device allowing these two quantities to be measured by means of sensors such as gyrometers and accelerometers, commonly being three in number of each type, disposed along three orthogonal axes, this set of three sensors forming an inertial measurement unit, commonly denoted IMU. The time integration of the acceleration data, and the projection into the navigation reference based on the speed of rotation data, allow the position and the speed of the carrier craft with respect to the Earth to be determined, with the knowledge of an initial state of these data. However, one drawback linked to the time integration is that the error associated with the data thus determined is an increasing function of time. This error increases more than linearly, typically exponentially, the variation of the error being commonly denoted drift of the inertial guidance system. Thus, for applications requiring a precise navigation, it is necessary to hybridize the inertial measurements with other measurements of position and/or speed and/or attitude of the carrier craft supplied by complementary sensors, such as baro-altimeters, odometers, Pitot probes, etc., with the goal of reducing the drift of the inertial guidance system. Such sensors supply information on the kinematic state of the carrier craft without requiring access to external signals or onboard maps, and are commonly denoted low-level sensors.
Radio-navigation consists in utilizing the signals coming from beacons transmitting radio signals, in order to extract information on positioning of the carrier craft with respect to these beacons. A radio-navigation technique that is widely used is the satellite geo-positioning technique, commonly denoted by the acronym GNSS corresponding to “Global Navigation Satellite System”, one representative of which is the GPS technique, corresponding to “Global Positioning System”. One of the drawbacks specific to radio-navigation techniques is linked to the fact that the reception of the signals coming from the beacons is not guaranteed at every place and time, and can notably be affected by the geophysical environment of the carrier craft, and also by the surrounding electromagnetic noise, where jamming techniques can notably compromise the operation of a radio-navigation device. Furthermore, since the transmitting beacons are maintained by operators, the integrity of the radio-navigation data coming from them is highly dependent on the cooperation of the latter. Radio-navigation, and notably the satellite geo-positioning system, and inertial navigation are for example complementary navigation techniques, and a hybridization of the two techniques can, in practice, result in a high-performance system. Inertial navigation indeed constitutes a very good local position estimator with a long-term drift, the satellite geo-positioning not being very reliable over the short term owing to the aforementioned drawbacks, but not exhibiting any drift. However, in the most critical applications, and notably for military applications, it is essential to turn to other sources of information on position and/or on speed and/or on attitude of the carrier craft in order to achieve hybridization with an inertial navigation technique. It is notably desirable that these alternative sources allow measurements of position and/or of speed and/or of attitude of the carrier craft which are independent, not subjected to jamming, and discrete.
Terrain Aided Navigation or TAN consists in utilizing geophysical data measurements delivered by a suitable sensor with reference data specific to a terrain covered by the navigation. The sensors are thus used in conjunction with a reference map of the terrain, also denoted onboard map. These sensors allow a data value characteristic of the terrain to be read, and the terrain aided navigation consists in comparing these values with the data of the onboard map, the onboard map being a prior sampling of the values of these data over the navigation region in question, obtained by suitable means, and henceforth denoted data production channel. Terrain Aided Navigation is particularly well adapted to hybridization with an inertial navigation technique, and allows the shortcomings of radio-navigation to be overcome. Of course, it is possible, for optimal performance, to use a navigation system allowing hybridization of the aforementioned three navigation techniques.
Generally speaking, any navigation system involving a terrain correlation thus comprises a plurality of onboard sensors comprised within the inertial guidance system, together with the terrain sensor, an onboard map representing the best possible knowledge on the reality of the geophysical data that the onboard sensor must measure, and a navigation filter. The navigation filter allows a judgment to be made, in real time, between the information supplied by the inertial guidance system and that supplied by the comparison between the measurements supplied by the terrain sensor and the onboard map. The judgment is made by the filter according to its prior knowledge of the errors on the measurements supplied. This knowledge is contained in error models. The error models relate to the inertial guidance system, the errors of the inertial guidance system being variable depending on the quality of the equipment; the error models also relate to the terrain sensor, together with the onboard map, the errors of the latter being variable depending on the quality of the data production channel. The error models for the equipment come from information supplied by the manufacturers, and/or come from measurements carried out via specific studies. The error models for the onboard maps are supplied by the data producers.
One essential aspect of the navigation is the stochastic nature of the phenomena being considered. Indeed, the sensors produce errors according to stochastic models and, since the knowledge of the geophysical data is not well controlled, the solution to the navigation problem using a filtering technique renders the navigational performance intrinsically stochastic. Thus, the filter used in a navigation system may be considered as an estimator of a stochastic process, which is to say as the device that provides, at any given moment, the dynamic state of the carrier craft modeled as a random variable.
A first example of a navigation system involving a terrain correlation is based on the technique of altimetric navigation. This technique consists in navigating a transport aircraft by means of an inertial guidance system, a terrain sensor of the radio-altimeter or multi-beam laser scanner type, measuring the distance between the carrier craft and the terrain in one or more given direction(s), and an onboard map of the Digital Terrain Model or DTM type, sampling the altitudes of points on the ground on a regular geo-localized grid.
A second example of a navigation system involving a terrain correlation is based on the bathymetric navigation technique. This technique consists in navigating a transport sea craft or submarine by means of an inertial guidance system, a terrain sensor of the single-beam or multi-beam bathymetric sounder type measuring the distance from the carrier craft to the seabed in one or more given direction(s), and an onboard map of the bathymetric map type sampling the altitudes of points on the seabed on a regular geo-localized grid.
A third example of a navigation system involving a terrain correlation is based on the technique of gravimetric navigation. This technique consists in navigating an aircraft, sea craft or submarine by means of an inertial guidance system, a terrain sensor of the gravimeter or accelerometer type measuring the local gravitational field or its anomaly, and an onboard map of the gravimetric anomaly map type sampling the values of the anomalies in the Earth's gravitational field at points of the globe on a normalized regular grid.
A fourth example of a navigation system involving a terrain correlation is based on the technique of navigation by vision. This technique consists in navigating an aircraft by means of an inertial guidance system, a terrain sensor of the onboard camera type which delivers images of the land over which it flies at a given frequency in the visible or infrared spectrum, and two onboard maps, one onboard map of the geo-localized ortho-image type, in other words an image that is re-sampled in such a manner that the effects of the mountainous areas have been removed, in other words for which the scale is the same at all the points, together with an onboard map of the DTM type.
In the framework of navigation systems involving a terrain correlation, the designers are notably confronted with a certain number of technical problems stated hereinbelow:                a navigation system must be defined that allows a desired quality of navigation according to a given set of performance criteria, for example guaranteeing a mean positioning error less than a given threshold, at a lower cost;        the most faithful error models possible for the inertial guidance system, the terrain sensor and the onboard map must be determined;        the missions of a carrier craft must be defined, notably in terms of input trajectory, during a mission preparation phase, in order to determine an optimal trajectory along which the quality of the signal delivered by the terrain sensor is maximized, where the optimal trajectory must also be defined with respect to other performance criteria for the carrier craft mission and to operational constraints associated with the theatre of the mission. The mission preparation phase must for example be based on a navigability criterion which is relevant, in other words representative of the richness of the signal delivered by the terrain sensor;        a high-performance navigation filter must be defined that is robust and capable of taking into consideration, at best, all the error models relating to the various components of the system, in other words the error of the inertial guidance system, of the terrain sensor and of the onboard map.        
The main object of the present invention is to solve the aforementioned technical problem, relating to the definition of a high-performance navigation filter. According to known techniques of the prior art, the navigation filters employed in TAN systems are navigation filters of the extended Kalman filter type, commonly denoted by the acronym EKF. These filters are notorious for not being robust in the case of a lack of information coming from the terrain, resulting in cases that can lead to a divergence of the filter. Known solutions allowing these divergences to be avoided consist in coupling EKF filters with block re-centering algorithms. However, typically, block re-centering phases can last of the order of twenty seconds to one minute, during which lapse of time no information on the behaviour of the system is returned. Such “silence” effects can have a detrimental impact on the navigation of a carrier craft, which can thus travel up to 15 km, for a speed of travel equal to 250 m/s, without any means of awareness of the quality of its navigation.
Non-linear filters that are more generic than Kalman filters are known from the prior art. These are particle filters which overcome the defects of EKF filters. Nevertheless, particle filters exhibit effects of degeneration after a relatively long navigation time. For this reason, particle filters have never been utilized in practice until now in navigation applications using terrain correlation.